Electron sources are particularly although not exclusively useful in display applications, especially flat panel display applications. Such applications include television receivers and visual display units for computers, especially although not exclusively portable computers, personal organisers, communications equipment, and the like. Flat panel display devices based on a magnetic matrix electron source of the present invention will hereinafter by referred to as Magnetic Matrix Displays.
UK Patent Application 2304981 discloses a magnetic matrix display having a cathode for emitting electrons, a permanent magnet with a two dimensional array of channels extending between opposite poles of the magnet, the direction of magnetisation being from the surface facing the cathode to the opposing surface. The magnet generates, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam. The display also has a screen for receiving an electron beam from each channel. The screen has a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of stripes per column, each stripe corresponding to a different channel. Flat panel display devices based on a magnetic matrix will hereinafter by referred to as Magnetic Matrix Displays.
The permanent magnet in a magnetic matrix display cannot be operated at the normal thermionic cathode temperature (993K) because this is beyond the Curie temperature--the point at which the magnet loses its magnetism properties. Methods for reflecting the majority of the thermionic cathode heat from the magnet have been previously disclosed, as have methods of heatsinking the magnet. However, it would be desirable if the cathode did not produce heat that needs to be either reflected and dissipated or dissipated by heatsinking.
Non-thermionic cathodes (i.e. so-called "cold" cathodes) are available. Examples are Metal-Insulator-Metal (MIM) cathodes, microtips and many others. However, these cathodes are all field emission types, characterised by the need for a strong electric field in the vicinity of the cathode material to pull electrons free from the cathode surface into the vacuum above the cathode. Two important characteristics of these cathodes make their use in a magnetic matrix display difficult:
1. The released electrons have a high eV. High electron energies will lead to the need for high Grid 1 voltages to ensure adequate differentiation between the "cut-off" and "non-select" levels. To obtain this, high voltage Gl drivers will be required, which are more costly than their low voltage counterparts. PA0 2. A very good vacuum is needed to prolong cathode life.
A third type of cathode--the photo-cathode--is known and can be used in this application. Electron emission from these is based on the photoelectric effect, that is, photons with sufficiently short wavelength (sufficiently high energy) can "knock" electrons free from the cathode material. Photo-cathodes are well known, being used for many decades in devices such as image intensifiers, film audio processing and the like.
Photo-cathodes fall into two categories--those lit from the front, and those lit from the rear. For magnetic matrix displays applications, a backlit photocathode is preferred. A preferred light source is the fluorescent tubes used in LCD backlights, with the lamp colour set to the point of maximum cathode efficiency. In order to obtain high (quantum) efficiency, at least one of the photo-cathode materials is picked to have a low work function e.g. Caesium (Cs) @ 1.4 V. Whilst this increases the quantum yield, the cathode surface is highly reactive and this makes fabrication of the cathode difficult for cases where it is fabricated in other than its place of use. For example, in a photomultiplier tube (PMT), the cathode materials are deposited on a wire filament. Once the PMT has been fabricated and evacuated, only then is the filament "fired" to deposit the cathode material on the inside of the top glass face of the tube. Typically the distance between the filament and working face of the tube is of the order of a few tens of mm.
Conventional methods of vapour deposition of a photo-cathode have relied on a small coil or coils disposed around the periphery of the active cathode area. The cathode material is deposited by heating these coils to evaporate off the photo-cathode materials placed on them during manufacture. In a magnetic matrix display, these coils cannot be in the active display area and thus they need to be placed around the periphery of the display area. This means that there is a substantial difference in distance between the coil to backplate distance at the edge of the display when compared with the coil to backplate distance at the centre of the display. Thus evaporation of the photo-cathode material across the desired cathode area will be highly nonuniform.
In the cathode region of a magnetic matrix display, space between the back plate of the display and the magnet assembly is limited. This means that conventional photo-cathode deposition techniques using a plurality of heater filaments cannot be applied whilst retaining a uniform layer of photoemissive materials. In view of the manufacturing difficulties associated with storing extremely reactive photo-cathodes, less reactive cathode materials may be used, but with lower quantum efficiency or reduced spectral response. An example of such a cathode system was described in Information Display magazine, August 1997--Vol. 13, No. 8. The energy of electrons emitted from a photo-cathode is nominally the difference between the photon's energy which causes the emission and the work function of the cathode material i.e. the energy the electron loses in escaping from the lattice. This is usually quite low, limited to a few eVs at most, and typically a few tenths of an eV. This makes the photo-cathode a preferred choice because of the low Grid 1 voltage which needs to be employed to hold inactive pixels at the "non-select" level when compared to active pixels at the "cut-off" level.
At least two of the problems which must be addressed for use of a photo-cathode in a magnetic matrix display are that it must be sufficiently efficient so as to reduce the overall power consumed by the display and that it must provide the required uniformity of emission.